Minor style cleanups in Cache code

- Make ApplicableDeclarationsCache::cache priv
- Add a missing semicolon for clarity
- Remove a useless `drop` call.
Minor code cleanup in css::matching
Fix deriving in cascade_with_cached_declarations

Even though we don't recompute non-inherited properties here, we still need
to compute properties that are derived from them.  Fixes #2498: because
text-decoration is not inherited, -servo-text-decorations-in-effect was not
derived correctly for cached declarations.
Drop event listener exceptions on the floor.

We get a lot of failures from assertions that event listeners haven't
thrown exceptions. As the FIXME above these assertions suggests, we
should just drop them on the floor.
auto merge of #2581 : mbrubeck/servo/declarations-cache, r=pcwalton

Fixes #2498.  Also includes a reftest, and some related code cleanup (in separate commits).

Note: The fix in `properties.rs.mako` is much more readable with `git diff -w`.
auto merge of #2582 : zwarich/servo/drop-exceptions, r=jdm

We get a lot of failures from assertions that event listeners haven't
thrown exceptions. As the FIXME above these assertions suggests, we
should just drop them on the floor.
Upgrade Rust.